1. Field of the Invention
The present invention relates to a joint assembly for plastic tubes.
2. Description of the Prior Art
With regard to a joint for plastic tubes, a great difficulty is encountered because the plastic tube is soft and has no ability to support the fastening force and, in addition, the deformation which is easily effected by a relatively small force is mostly plastic thereby excluding any expectation for the elastic packing function by the tube itself. Furthermore, since the softness and the plasticity of plastic tubes are increased as the temperature thereof becomes higher, a greater difficulty is encountered in the joint for plastic tubes used with a high temperature fluid or in a high temperature atmosphere.
Conventionally, a joint assembly which is commonly used for plastic tubes comprises a sleeve to be inserted into an end portion of a plastic tube for supporting the inner peripheral surface of said end portion, an annular caulking element having a substantially triangular cross section as a whole and adapted to be mounted around said end portion, the body member having an annular seat for receiving a tapered outer peripheral surface of said annular caulking element, and a cap nut having an annular seat for receiving another tapered outer surface of said annular caulking element and adapted to be screwed onto said body member. In this conventional joint assembly, wherein said sleeve is formed either as a unitary portion of the body member to project from an end portion thereof or as an independent element because it is relatively difficult to form the unitary sleeve with a body member, as the cap nut is screwed onto the body member, opposite end portions of said tapered outer peripheral surfaces of said annular caulking element are urged gradually inwardly due to engagement with said annular seat of the body member so that said opposite end portions cut into the outer peripheral surface of the plastic tube supported by said sleeve at its inner peripheral surface thereby establishing a gas-tight connection for the terminal of the plastic tube. In this conventional joint assembly, however, although a relatively good gas-tight connection is obtained in an early period of fastening by the opposite end portions of the annular caulking element being elastically caulked into the outer peripheral surface of a plastic tube, the plastic tube soon experiences plastic deformation at its pressed portion. This deformation is especially rapid when the tube is subjected to a relatively high temperature, whereby the contact pressure at the caulked portion gradually lowers thereby increasing the danger that leakage occurs at the contacting portion. The leakage due to the plastic deformation at the fastened contacting portion becomes remarkable when the plastic tube is used with a hot fluid or in a hot atmosphere to such an extent that the conventional joint assembly of this type is almost inapplicable for high temperature services.